This invention relates to an electrostatic latent image developer comprising a magnetic toner and a carrier and a method for developing an electrostatic latent image using the developer.
For the development of electrostatic latent images, mono component developers using magnetic toner are well known in the art. Triboelectric magnetic toners comprising a magnetic toner and a charge control agent are also known as disclosed in Japanese Patent Application Kokai Nos. 48754/1980, 45555/1982, 45556/1982, and 45557/1982. These mono component toners suffer from agglomeration due to static charges which causes image defects such as white streaks.
Techniques for preventing such toner agglomeration are disclosed in Japanese Patent Application Kokai Nos. 121054/1984, 182464/1984, 210450/1984, 210466/1984, 216149/1984, 42163/1987, 275280/1987, and 294259/1987. These developing compositions are prepared by adding a carrier to a triboelectric magnetic toner having internally added thereto a charge control agent, for example, a chromium complex of a monoazo dye such as Bontron S-34 (manufactured by Orient Chemical K.K.) and a Nigrosine dye such as Bontron N-01 (manufactured by Orient Chemical K.K.). The carrier is added in amounts of 60 to 90% by weight and 30 to 90% by weight in Japanese Patent Application Kokai Nos. 42163/1987 and 294259/1987, respectively. Japanese Patent Application Kokai No. 182464/1984 indicates to add minor amounts of carrier.
In these patent applications, it is generally believed that the composition should be carrier rich in order to prevent toner scattering, to reduce the amount of toner spent, and to extend the life of carrier. For this reason, more than 50% by weight of carrier is mixed with the toner in all the examples of these patent applications.
Japanese Patent Application Kokai No. 162563/1984 discloses an example in which a developing composition is prepared by adding 90 to 60% by weight of a carrier to a triboelectric magnetic toner having internally added thereto a charge control agent in the form of Aizen Spilon Black TRH (manufactured by Hodogaya Chemical K.K.) which is a monoazo dye chromium complex. It also discloses a comparative example in which a developing composition is prepared by adding 40% by weight of a carrier to a similar triboelectric magnetic toner, which is reported to undergo background fogging due to toner scattering and a lowering of copy image density.
In the developing compositions of Japanese Patent Application Kokai No. 182464/1984 and other patent applications, the initial load of carrier must be increased. During continuous printing, the toner is replenished whereupon the toner is not immediately distributed over the carrier, resulting in reduced image density and failing to reproduce copies of constant quality.
If the initial load of toner is increased in such developing composition, the content of carrier would be 30 to 40% by weight or lower. With the use of a triboelectric toner having internally added thereto a monoazo dye metal complex or a Nigrosine dye, the reduced carrier content would cause background fogging due to toner scattering and a reduction in copy image density.